tube or a split cylinder. On the other hand, in the external loop, riser and downcomer are separated physically by two different tubes.
3.1.1 Major Factors Affecting the Vertical PBRs Performance
Light Available
In vertical photobioreactors, the illumination is accomplished externally, which can
be natural or artificial. The light available plays an essential role for the good
performance of any photosynthetic culture. To obtain sufficient illumination, both
the airlift and the bubble column photobioreactors cannot exceed about 0.2 m in
diameter; otherwise, the light availability will be reduced severely, mainly in the
center of cylinder. Additionally, it must also be considered that the height of the
cylinder should not exceed 4 m due to structural reasons (Huang et al. 2017).
Furthermore, in a vertical tubular photobioreactor, the light availability also is
influenced by aeration rates, gas holdup, and superficial velocity (Mirón et al.
1999).
Aeration Rate, Gas Holdup, and Superficial Gas Velocity
As in horizontal photobioreactors, the agitation of the system gives only by
pneumatic path, the aeration is responsible for culture mixing. In an ideal aeration
rate, the microalgae are kept in suspension, the light/dark cycle is minimized, the
CO 2 is diffused homogeneously, and its excess is removed, thus maintaining the pH
stable and the produced oxygen is removed. In general, in airlift photobioreactors,
the mixing is better than bubble column and thus can sustain better biomass production of different microalgae (Fernandes et al. 2014).
Gas holdup is one of the most important parameters characterizing airlift and
tubular photobioreactors. It is necessary to the hydrodynamic design in different
industrial processes because it governs gas phase residence time and gas–liquid
mass transfer. The gas holdup is defined as the volume of the gas phase divided by
the total volume. This parameter is influenced mainly by the superficial gas velocity
and the type of gas diffuser (Mirón et al. 1999).
Superficial gas velocity is the ratio of the volumetric gas flow rate and
cross-sectional area of the reactor. The photosynthetic efficiency of the culture is
affected by the dark zone that may exist at the center of the photobioreactor. This
dark zone is totally dependent on the gas superficial velocity, which is the ratio of
the volumetric gas flow rate and cross-sectional area of the reactor. According to
Janssen et al. (2003), high gas velocity (>0.05 m/s) is recommended for increasing
the photosynthetic efficiency.
2 Microalgal Production Systems with Highlights …
19
3.1.1 Major Factors Affecting the Vertical PBRs Performance
Light Available
In vertical photobioreactors, the illumination is accomplished externally, which can
be natural or artificial. The light available plays an essential role for the good
performance of any photosynthetic culture. To obtain sufficient illumination, both
the airlift and the bubble column photobioreactors cannot exceed about 0.2 m in
diameter; otherwise, the light availability will be reduced severely, mainly in the
center of cylinder. Additionally, it must also be considered that the height of the
cylinder should not exceed 4 m due to structural reasons (Huang et al. 2017).
Furthermore, in a vertical tubular photobioreactor, the light availability also is
influenced by aeration rates, gas holdup, and superficial velocity (Mirón et al.
1999).
Aeration Rate, Gas Holdup, and Superficial Gas Velocity
As in horizontal photobioreactors, the agitation of the system gives only by
pneumatic path, the aeration is responsible for culture mixing. In an ideal aeration
rate, the microalgae are kept in suspension, the light/dark cycle is minimized, the
CO 2 is diffused homogeneously, and its excess is removed, thus maintaining the pH
stable and the produced oxygen is removed. In general, in airlift photobioreactors,
the mixing is better than bubble column and thus can sustain better biomass production of different microalgae (Fernandes et al. 2014).
Gas holdup is one of the most important parameters characterizing airlift and
tubular photobioreactors. It is necessary to the hydrodynamic design in different
industrial processes because it governs gas phase residence time and gas–liquid
mass transfer. The gas holdup is defined as the volume of the gas phase divided by
the total volume. This parameter is influenced mainly by the superficial gas velocity
and the type of gas diffuser (Mirón et al. 1999).
Superficial gas velocity is the ratio of the volumetric gas flow rate and
cross-sectional area of the reactor. The photosynthetic efficiency of the culture is
affected by the dark zone that may exist at the center of the photobioreactor. This
dark zone is totally dependent on the gas superficial velocity, which is the ratio of
the volumetric gas flow rate and cross-sectional area of the reactor. According to
Janssen et al. (2003), high gas velocity (>0.05 m/s) is recommended for increasing
the photosynthetic efficiency.
2 Microalgal Production Systems with Highlights …
19